1. Field of the Invention
The invention relates to computer-based systems that scan hand-drawn drawings into a computer and process them. More particularly, it relates to systems that scan hand-drawn cartoon animation and prepare the scanned drawings for computer-performed processing, including inking, painting, and camera tasks.
2. Background
The traditional manual process for preparing cartoon animation begins with hand-drawn animation, typically pencil on paper. The drawings then go through a three-step process before they appear on film.
The first step is "ink". The lines of the pencil drawings are traced onto the front of clear plastic acetate sheets (called cels) with one or more colors of ink. If a completely black line is acceptable, the drawings are often photocopied onto the cel. In either case, the result is a solid, opaque line.
The second step is "paint". After the front of the cel is inked, the drawing is filled in on the back of the cel with various colors of paint.
The third step is "camera". All the painted cels for a particular frame are stacked and photographed to generate the final frame.
This traditional, three-step, manual process of inking and painting cels has a number of drawbacks. It is very labor intensive. It is difficult to avoid painting errors, such as painting a region the wrong color on one cel, and painting errors are time consuming to fix. It is very labor intensive to make a widespread color change. The cels are not completely transparent, so colors on lower cels in a final frame appear darker and desaturated. And a cel will get dusty and dirty, and that dirt appears visually in the final frame if the cel is not cleaned each time it is used.
In recent years, several computer-based systems have been developed to address the drawbacks of the manual ink-and-paint process. One such system is the USAnimation System, available from USAnimation, Inc. of Hollywood, Calif. Generally, such systems scan the original pencil drawings into the computer, and then perform the ink, paint, and camera tasks on the computer. The USAnimation product reduces the line art to vectors representing the center of the line art. These vectors--the "color art" --are found by pixel operations, using three separate copies of the image pixmap. With a drawing on standard 16 by 12 inch paper scanned at 300 dots per inch, the size of each pixmap exceeds 17 million bits. The line art polygons are then formed as a set of vector polygons, each polygon being formed around one unbroken, branchless section of the color art. The result of this process is illustrated in FIG. 1A, which shows a drawn "Y" digitized into a bitmap, and FIG. 1B, which shows the three regions of line art 102, 104, and 106 created by the USAnimation product from the bitmap of FIG. 1A.
Many such systems scan the pencil drawing as a pixmap with various levels of gray, so the scanned line has a soft edge. A "pixmap" is a data structure storing an image in pixel representation. A pixel is a picture element; if one thinks of a picture as a raster of points, a pixel is a point. In a pixmap, a pixel will have one or more bits of information, the number of bits being referred to as the "depth", which bits may represent gray scale or color information.
Generally, the lines of the pencil drawing in the pixmap are recolored to reflect the required ink color, and the appropriate regions of the pixmap are painted. Then, these digital cels are layered in the computer through a digital compositing process and written to an appropriate medium, such as video tape or film.
Such a digital process has a number of advantages over the manual process. It is less labor-intensive because computer paint programs allow for greater productivity than processes using physical ink and paint. Fixing paint errors on a computer is quick and easy. Widespread color changes in ink or paint can be implemented with minimal labor. Digital cels are completely transparent, so there are no problems with color shifts, and therefore no inherent limit in the complexity of the cartoon. Dirt specks picked up in the scanning process are easily eliminated from the digital cels and never reappear.
However, in such systems, a new set of issues arises and some old problems still exist.
First, the resolution of the pixmap is determined at scanning time. In order to avoid digital artifacts, it is necessary that the scanning resolution for a cel be as large or larger than the output resolution multiplied by the most extreme truck (or amount of zoom in) for that cel.
Second, the effort and computer resources required to ink and to paint increase with the scanning resolution.
Third, painting errors are still difficult to prevent and detect. The pixmap for a digital cel is relatively large, requiring a significant amount of computer memory in the paint program. If there is not enough memory available to store more than a few cels at a time in the paint program's memory, it may be difficult to see the cartoon animate in order to detect painting errors.
And fourth, the ink may retain the variable density of the original pencil line in subsequent digital processing, a departure from the look of traditional animation.